Hydrogen adsorption structures in magnesium calcite

In this paper, we report the hydrogen adsorption structure of magnesium calcite, a naturally occurring calcium carbonate sample treated with hydrogen at high pressure and temperature. The hydrogen adsorption structure has been studied by investigating the formation of hydrides and structural changes occurring before and after the treatment of the raw material and in the temperature range of 100–800 °C. The maximum hydrogen release occurred at a temperature of 700 °C. X-ray diffraction, X-ray photoelectron spectroscopy, and gas chromatography results suggested that MgH 2 increased hydrogen release. Additionally, the volume of the gas released was not observed to be linearly proportional to hydrogen desorption. Further, the hydrogen adsorption structure of magnesium calcite demonstrated that the MgH 2 phase did not precipitate and was adsorbed as a single MgH 2 molecule at the interstitial sites of calcite. Notably, the number of Mg 2 + ion substitutions per unit lattice of magnesium calcite was assumed to be discrete. • MgH2 formation increased hydrogen release in magnesium calcite. • Maximum hydrogen release (1093 ppm) occurred at 700 °C. • MgH2 adsorbed as a single molecule at calcite interstitial sites. • Hydrogen release was not linearly proportional to hydrogen desorption. • Discrete Mg²⁺ ion substitutions affect adsorption in calcite.